“Apoptosis” refers to cell suicide that proceeds by an active, physiological process (Kerr, J. F., et al., Br. J. Cancer 26:239–257 (1972); Wyllie, A. H., Nature 284:555–556 (1980)). Apoptosis plays an important role in developmental processes, including morphogenesis, maturation of the immune system, and tissue homeostasis whereby cell numbers are limited in tissues that are continually renewed by cell division (Ellis, R. E., et al., Annu. Rev. Cell. Biol. 7:663–698 (1991); Oppenheim, R. W., et al., Neurosci. 14:453–501 (1991); Cohen, J. J., et al., Annu. Rev. Immunol. 10:267–293 (1992); Raff., M. C., Nature 356:397–400 (1992)).
In addition to its role in developmental processes, apoptosis is an important cellular safeguard against tumorigenesis (Williams, G. T., Cell 65:1097–1098 (1991); Lane, D. P. Nature 362:786–787 (1993)). Defects in the apoptotic pathway may contribute to the onset or progression of malignancies. Suppression of the apoptotic pathway(s), by a variety of genetic lesions, occurs frequently in a broad range of human tumors. In particular, loss of the p53 tumor suppressor gene function, either through deletion or mutation, occurs in more than 50% of human cancers. p53 gene function is also indicated in normal cell cycle events. Reviews of p53 function include Levine, A. J., et al., Nature 351:453–456 (1991); Hollstein M., et al., Science 253:49–53 (1991); Donehower, et al., Biochem. BioPhys. Acta 1155:181–205 (1993); Lane, D. P. Nature 362:786–787 (1993); Zambetti, et al., FASEB J. 7:855–865 (1993); and Greenblatt M. S., et al., Cancer Res., 54: 4855–4878 (1994).
p53 may exert its tumor suppressor function, at least in part, by directing cells that have sustained genomic damage to undergo apoptosis (Lowe S. W., Jacks T., Housman D. E. and Ruley H. E. (1994) Proc. Natl. Acad. Sci. USA, 91, 2026–2030). p53 is a sequence-specific DNA binding protein that functions both as a transcriptional activator and repressor (Donehower L. A. and Bradley A. (1993) Biochim. Biophys. Acta., 1155, 181–205; Prives C. and Manfredi J. (1993) Genes Dev., 7, 529–534; Fields S. and Jang S. K. (1990) Science, 249, 1046–1048; Raycroft L., Wu H. and Lozano G. (1990) Science, 249, 1049–1051). Although there is some evidence that transcription may not be required in p53-mediated apoptosis (Caelles C., Helmberg A. and Karin M. (1994) Nature, 370, 220–223), several p53-regulated genes have been identified to date (Kastan M. B., Zhan Q., El-Deiry W. S., Carrier F., Jacks T., Walsh W. V., Plunkett B. S., Vogelstein B. and A. J. Fornace Jr. (1992) Cell, 71, 587–597, 1992; El-Deiry W. S., Tokino T., Velculescu V. E., Levy D. B., Parsons R., Trent J. M., Lin D., Mercer W. E., Kinzler K. W. and Vogelstein B. (1993) Cell, 75, 817–825; Barak Y., Juven T., Haffner R. and Oren M. (1993) EMBO J., 12, 461–468; Wu X., Bayle J. H., Olson D. and Levine A. J. (1993) Genes & Dev., 7, 1126–1132; Zambetti G. P., Bargonetti J., Walker K., Prives C. and Levine A. J. (1992) Genes & Dev., 6, 1143–1152; Okamoto K. and Beach D. (1994) EMBO J, 13, 4816–4822; Buckbinder L., Talbott R., Seizinger B. R. and Kley N. (1994) Proc. Natl. Acad. Sci. USA, 91, 10640–10644, and two of these genes, bcl-2 and bax (Miyashita T. and Reed J. (1995) Cell, 80, 293–299; Miyashita T., Krajewski S., Krajewska M., Wang H., Lin H., Hoffman B., Lieberman K. and Reed J. (1994) Oncogene, 9, 1799–1805; Zhan Q., Fan S., Bae I., Guillouf C., Liebermann D. A., O'Connor P. M. and A. J. Fornace Jr. (1994) Oncogene, 9, 3743–3751), have been clearly implicated in apoptosis (Oltvai Z. and Korsmeyer S. (1994) Cell, 79, 189–192).
In addition to cancer, deregulation of apoptosis may contribute to a number of other human diseases. A variety of degenerative disorders may involve aberrant apoptosis, resulting in premature or inappropriate cell death (Barr, P. J., et al., Biotechnology 12:487–493 (1994)) Productive infection by certain viruses may depend on suppression of host cell death by anti-apoptotic viral gene products (Rao, L., et al., Proc. Natl. Acad. Sci. USA 89:7742–7746 (1992); Ray, C. A., et al., Cell 69:597–604 (1992); White, E., et al., Mol. Cell. Biol. 12:2570–2580 (1992); Vaux, D. L., et al., Cell 76:777–779 (1994), and inhibition of apoptosis can alter the course (i.e. lytic vs. latent) of viral infection; Levine, B., et al., Nature 361:739–742 (1993)). Widespread apoptosis of T lymphocytes triggered by HIV infection may, at least in part, be responsible for the immune system failure associated with AIDS (Gougeon M., et al., Science 260:1269–1270 (1993)).
The ability of p53 to suppress tumorigenesis appears linked to its activity as a transcriptional activator, since tumor-derived mutant p53 molecules almost invariably have lost transactivation potential (Kern, S. E., et al., Science 256:827–830 (1992)). Thus, the function of the p53 tumor suppressor appears to depend, at least in part, on the ability to activate the expression of one or more target genes. Genes activated by p53 may in turn mediate one or more aspects of p53's tumor suppressor function, which including cell cycle arrest and apoptosis, depending on the cellular context. Consistent with the notion, certain p53-activated genes identified to date have been implicated in cell cycle control (gadd45, cyclin G, p21/WAF) and at least one p53-activated gene (bax) is linked to the regulation of apoptosis.
Tumor cells frequently have lost wild-type p53 function. Consequently, activation of p53 target genes and associated tumor suppressor functions, such as cell cycle arrest and apoptosis, is defective in cancer cells. Therefore, from the perspective of pharmaceutical development, identification of genes which are regulated (e.g, induced or repressed) by p53 may permit development of agents that activate, restore or suppress p53-dependent tumor suppression functions such as apoptosis or cell cycle regulation, depending on the clinical setting.